Use goal preview metadata for goal-first threads (#21981)

Fixes #20792

## Why

`/goal`-first threads are valid resumable threads, but they can be
missing from `codex resume` and app recents because discovery depends on
metadata derived from a normal first user message.

PR #21489 attempted to fix this by using the goal objective as
`first_user_message`. Review feedback pointed out that
`first_user_message` does more than provide visible text today: it gates
listing, supplies preview text, and participates in deciding whether a
later title should surface as a distinct thread name. Reusing it for the
goal objective could leave a `/goal`-first thread with
`first_user_message=<goal>` and `title=<later prompt>`, even though the
goal should only provide the initial visible preview.

This PR follows that feedback by and keeps the `first_user_message` as
is but introduces a new `preview` field to separate concerns. The
`preview` field is populated from the first user message or the goal
objective. We can extend it in the future to include other sources.

## What Changed

- Added internal thread `preview` metadata in `codex-state`, including a
SQLite migration that backfills from `first_user_message` and from
existing `thread_goals` objectives when needed.
- Treated `ThreadGoalUpdated` as preview-bearing metadata so goal-first
threads can be listed and searched without mutating
`first_user_message`.
- Updated rollout listing, state queries, thread-store conversion, and
app-server mapping to use preview metadata while continuing to expose
the existing public `preview` field.
- Preserved title/name distinctness behavior around literal
`first_user_message`, so a later normal prompt after `/goal` does not
surface as a separate name just because the goal supplied the initial
preview.
- Preserved compatibility for older/internal metadata writes by deriving
preview from `first_user_message` when explicit preview metadata is
absent.

## Verification

- Manually verified that a thread that starts with a `/goal <objective>`
shows up in the resume picker.

codex-rs/rollout/src/list.rs

@@ -50,6 +50,8 @@ pub struct ThreadItem {
     pub thread_id: Option<ThreadId>,
     /// First user message captured for this thread, if any.
     pub first_user_message: Option<String>,
+    /// Best available user-facing preview for discovery and list display.
+    pub preview: Option<String>,
     /// Working directory from session metadata.
     pub cwd: Option<PathBuf>,
     /// Git branch from session metadata.
@@ -85,9 +87,9 @@ pub type ConversationsPage = ThreadsPage;
 #[derive(Default)]
 struct HeadTailSummary {
     saw_session_meta: bool,
-    saw_user_event: bool,
     thread_id: Option<ThreadId>,
     first_user_message: Option<String>,
+    preview: Option<String>,
     cwd: Option<PathBuf>,
     git_branch: Option<String>,
     git_sha: Option<String>,
@@ -738,7 +740,8 @@ async fn build_thread_item(
     cwd_filters: Option<&[PathBuf]>,
     updated_at: Option<String>,
 ) -> Option<ThreadItem> {
-    // Read head and detect message events; stop once meta + user are found.
+    // Read head and detect preview-bearing events; goal previews can appear before
+    // the first normal user message.
     let summary = read_head_summary(&path, HEAD_RECORD_LIMIT)
         .await
         .unwrap_or_default();
@@ -764,11 +767,12 @@ async fn build_thread_item(
     {
         return None;
     }
-    // Apply filters: must have session meta and at least one user message event
-    if summary.saw_session_meta && summary.saw_user_event {
+    // Apply filters: must have session meta and a discoverable preview.
+    if summary.saw_session_meta && summary.preview.is_some() {
         let HeadTailSummary {
             thread_id,
             first_user_message,
+            preview,
             cwd,
             git_branch,
             git_sha,
@@ -789,6 +793,7 @@ async fn build_thread_item(
             path,
             thread_id,
             first_user_message,
+            preview,
             cwd,
             git_branch,
             git_sha,
@@ -1078,7 +1083,7 @@ async fn read_head_summary(path: &Path, head_limit: usize) -> io::Result<HeadTai
 
     while lines_scanned < head_limit
         || (summary.saw_session_meta
-            && !summary.saw_user_event
+            && (summary.preview.is_none() || summary.first_user_message.is_none())
             && lines_scanned < head_limit + USER_EVENT_SCAN_LIMIT)
     {
         let line_opt = lines.next_line().await?;
@@ -1131,19 +1136,23 @@ async fn read_head_summary(path: &Path, head_limit: usize) -> io::Result<HeadTai
                 // Not included in `head`; skip.
             }
             RolloutItem::EventMsg(ev) => {
-                if let EventMsg::UserMessage(user) = ev {
-                    summary.saw_user_event = true;
-                    if summary.first_user_message.is_none() {
-                        let message = strip_user_message_prefix(user.message.as_str()).to_string();
-                        if !message.is_empty() {
-                            summary.first_user_message = Some(message);
-                        }
+                if let Some(preview) = event_msg_preview(&ev) {
+                    if summary.preview.is_none() {
+                        summary.preview = Some(preview.clone());
+                    }
+                    if let EventMsg::UserMessage(_) = ev
+                        && summary.first_user_message.is_none()
+                    {
+                        summary.first_user_message = Some(preview);
                     }
                 }
             }
         }
 
-        if summary.saw_session_meta && summary.saw_user_event {
+        if summary.saw_session_meta
+            && summary.preview.is_some()
+            && summary.first_user_message.is_some()
+        {
             break;
         }
     }
@@ -1198,6 +1207,31 @@ fn strip_user_message_prefix(text: &str) -> &str {
     }
 }
 
+fn event_msg_preview(event: &EventMsg) -> Option<String> {
+    match event {
+        EventMsg::UserMessage(user) => {
+            let message = strip_user_message_prefix(user.message.as_str());
+            if !message.is_empty() {
+                return Some(message.to_string());
+            }
+            if user
+                .images
+                .as_ref()
+                .is_some_and(|images| !images.is_empty())
+                || !user.local_images.is_empty()
+            {
+                return Some("[Image]".to_string());
+            }
+            None
+        }
+        EventMsg::ThreadGoalUpdated(event) => {
+            let objective = event.goal.objective.trim();
+            (!objective.is_empty()).then(|| objective.to_string())
+        }
+        _ => None,
+    }
+}
+
 /// Read the SessionMetaLine from the head of a rollout file for reuse by
 /// callers that need the session metadata (e.g. to derive a cwd for config).
 pub async fn read_session_meta_line(path: &Path) -> io::Result<SessionMetaLine> {